package standard library
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Published: Jun 13, 2024 License: BSD-3-Clause Imports: 12 Imported by: 163,509



Package rand implements a cryptographically secure random number generator.




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var Reader io.Reader

Reader is a global, shared instance of a cryptographically secure random number generator.

  • On Linux, FreeBSD, Dragonfly, and Solaris, Reader uses getrandom(2) if available, and /dev/urandom otherwise.
  • On macOS and iOS, Reader uses arc4random_buf(3).
  • On OpenBSD and NetBSD, Reader uses getentropy(2).
  • On other Unix-like systems, Reader reads from /dev/urandom.
  • On Windows, Reader uses the ProcessPrng API.
  • On js/wasm, Reader uses the Web Crypto API.
  • On wasip1/wasm, Reader uses random_get from wasi_snapshot_preview1.


func Int

func Int(rand io.Reader, max *big.Int) (n *big.Int, err error)

Int returns a uniform random value in [0, max). It panics if max <= 0.

func Prime

func Prime(rand io.Reader, bits int) (*big.Int, error)

Prime returns a number of the given bit length that is prime with high probability. Prime will return error for any error returned by rand.Read or if bits < 2.

func Read

func Read(b []byte) (n int, err error)

Read is a helper function that calls Reader.Read using io.ReadFull. On return, n == len(b) if and only if err == nil.


This example reads 10 cryptographically secure pseudorandom numbers from rand.Reader and writes them to a byte slice.

package main

import (

func main() {
	c := 10
	b := make([]byte, c)
	_, err := rand.Read(b)
	if err != nil {
		fmt.Println("error:", err)
	// The slice should now contain random bytes instead of only zeroes.
	fmt.Println(bytes.Equal(b, make([]byte, c)))




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